1. Field of the Invention
The present invention relates to the construction for an electrolytic furnace, and more particularly to an improved lining for an electrolytic furnace used to produce aluminum from aluminum chloride dissolved in molten salts of higher electrodecomposition potential.
2. Description of the Art
Electrolytic production of light metals is disclosed generally by Haupin in U.S. Pat. No. 3,755,099 as a process performed in a furnace or cell which includes at least two opposed electrodes providing at least one interelectrode space therebetween. More particularly, such cell includes an anode, at least one intermediate bipolar electrode and a cathode in superimposed, spaced relationship defining a plurality of interelectrode spaces therebetween. In such process a light metal chloride, such as aluminum chloride, is dissolved in a bath of molten salts having higher electrodecomposition potential than the light metal chloride. The bath is preferably maintained above the liquidus temperature of the light metal being produced. In the operation of the cell, chlorine is produced on the anode surface, and light metal, such as aluminum, is produced on the cathode surface. The process also provides for the maintenance of bath flow through each interelectrode space.
Although the lining disclosed in the present specification is partially directed to a furnace used for the production of aluminum from aluminum chloride, it is intended to be equally applicable to furnaces used to produce other metals from their metal chlorides, including magnesium from magnesium chloride, zinc from zinc chloride and lead from lead chloride.
A typical composition in weight percent for the bath in a cell from which aluminum may be produced by electrolysis is made up of about 51% sodium chloride (NaCl), about 40% lithium chloride (LiCl), about 6.5% aluminum chloride (AlCl.sub.3) and about 2.5% magnesium chloride (MgCl.sub.2). Other chlorides may be regarded as incidental components or impurities. Such bath is maintained in a molten state at a temperature above the melting temperature of aluminum of 660.degree. C.
A primary problem attendant the economic commercial production of aluminum by the electrolytic process described above is to contain the high temperature, corrosive bath constituents without detrimentally cooling the interior portions of the furnace. This problem is two-fold as it involves containment of not only the molten salt, but also the molten metal produced in the cell.
It has been found that the corrosive bath constituents react with certain cell lining materials and thereby cause bath contamination and, perhaps, premature consumption of the anodes. For example, silicon from silica-based refractories tends to contaminate the molten aluminum being produced. Also, refractories having high oxygen values may cause a reaction with carbon in the anodes to produce carbon monoxide and carbon dioxide, while consuming the anodes. Jacobs in U.S. Pat. No. 3,785,941 discloses the use of nitride-based refractory which does not react with the salts to corrode the refractory or contaminate the bath. Those skilled in the art also recognize the high cost involved in using such alternative refractory materials.
The majority of the molten aluminum formed in the cell is contained in a carbonaceous sump located in the lower portion of the cell. This sump is usually made of graphite which resists attack and penetration by molten aluminum. Portions of the molten aluminum, however, penetrate the sump or the interstices within the sump, and proceed into the refractory lining. Such penetration occurs primarily in the bottom or floor of the cell where the driving force is gravity. If the molten aluminum penetrates unimpeded through the refractory lining to the metal shell, at least a portion of the electrical system will short circuit. It is therefore desirable to stop the flow of molten aluminum through the refractory lining of an electrolysis cell to optimize production and energy efficiency thereof by preventing the possibility of short circuiting caused as a result of molten metal penetration.
The molten salt in the cell is continuously circulating through the cell, usually at temperatures approximating 700.degree. C. There is no known economically feasible material in the class of an insulating refractory that is impervious to such high temperature molten salt bath and is able to withstand molten metal attack. Therefore, the desired approach has been to utilize materials in the hot, inner layers of electrolysis cell linings that resist chemical corrosion when penetrated by the molten salts and the molten metal. It is well understood that, as the salts permeate the refractory lining of a cell, heat loss increases. At lower temperatures certain materials can be employed in the outer layers that better resist molten salt attack. For example, Russell et al in U.S. Pat. Nos. 3,773,643 and 3,779,699 teach that lower temperature molten salt penetration is inhibited by a layer of glass, such as window glass.
There are inorganic rigid glass foam materials that are not only impervious to lower temperatures molten salts but also exhibit thermal conductivities and thermal shock resistance that render these materials ideal for use in electrolysis cell linings. It will be understood that such materials can be constructed with higher or lower upper use temperature limitations with higher and lower cost, respectively. In an electrolysis cell lining, the greater thermal protection is required in the inner layers, while less thermal protection is required in the outer layers where pentrating salts have experienced heat loss.
Although certain materials are impervious to salt penetration at certain temperatures, such materials are constructed in brick or block form and laid in the lining. A portion of the molten salts readily flows through the seam or interface between adjacent bricks and proceeds outwardly toward adjacent layers of refractory. Also, although the salts experience significant heat loss as they penetrate the cell lining, and although the majority of the salt freezes within the lining, certain eutectics are formed with the aluminum chloride, for example, which have melting points less than about 100.degree. C. and are, therefore, highly penetrable.
Since the metal shell surrounding the cell exhibits a temperature that may be in excess of about 100.degree. C., at least a portion of the molten salts is not solidified or frozen within the lining. In any event, the salts must not be permitted to contact the metallic shell, otherwise chlorine is evolved at such anodic locations or aluminum is evolved at cathodic locations. The chlorine at anodic locations could decompose the steel shell. At the side walls, which are usually water cooled, coolant could flow through such holes into the cell.
U.S. Patent No. 4,140,595 of Russell et al, discloses the use of electrically insulative coatings on the inside surface of the metal shell which prevent molten salts at relatively low temperatures from penetrating therethrough and contacting the shell. Such coatings may be natural or synthetic rubber, asphalt or synthetic plastic, including polytetrafluoroethylene, silicone resins or epoxy resins. Since such coating materials are resistant to molten salts at low temperatures, it is important that the amount of salt reaching the coating area be reduced, and that the temperature of such penetrating salt be reduced below such temperature limit.
Thermal balance is critical in designing a lining for an electrolytic cell. The lining must not degrade to assure that the temperatures at any location within the cell lining remain substantially constant throughout the operation of the cell. In determining such balance it is imperative to consider salt penetration which usually raises the thermal conductivity of the penetrated refractory. Salt penetration, therefore, should be held relatively stable throughout the operation of the cell so as not to upset the established thermal balance. To assure stability and thermal balance, design parameters should be such that salt and metal penetration is controlled and that the lining does not degrade throughout the campaign of the cell.
Accordingly, an improved, economical lining is desired for an electrolytic cell which will minimize bath contamination and anode consumption and maximize the life of the furnace lining by maintaining a stable thermal balance therethrough.